At present, carriers of balanced non-Robertsonian translocations are usually given the same general emperic risk of between 10 and 20 percent of having a congenitally abnormal child as a direct result of the carrier having produced chromosomally unbalanced gametes. This implies that reciprocal translocations can be treated as a homgeneous class. However, it is known from studies of many organisms including man that the behavior of translocated chromosomes during meiosis depends on a number of factors including the relative size and centromere position of the chromosomes involved, the lengths of the interchanged segments, and the frequency and distribution of chiasmata between the rearranged chromosome pairs. It is the purpose of this cytogenetic study, using data collected from a large number of families is which a balanced translocation is segregating, to: 1) classify human reciprocal translocations according to their structural characteristics; 2) determine, by testing hypotheses based on observations in other organisms, what factors influence the orientation of quadrivalents during human meiosis, and thereby influence the type of segregation that occurs; and 3) establish more accurate risk estimates than are now available for carriers of specific classes of reciprocal translocations. The primary sources of data on the segregation of balanced translocations will be unpublished cases personally available to the principal investigator and published cases in which family studies have been done. Unpublished cases can be collected by using the established chromosome registries. A Univac 1110 Computer will be used in storing and analyzing the cytogenetic and family data collected in this indirect study.